It is generally known from the prior art to connect a cab of a vehicle in a tiltable manner to the chassis of the vehicle, so that the cab can be tilted between a driving position, in which the vehicle can be driven, and a tilted position, in which, for example, maintenance can be carried out on the engine situated below the cab. It is further generally known for such a tilting cab in the driving position to be resiliently supported on the chassis in order to provide greater comfort for those in the cab, in particular the driver, while the vehicle is being driven.
For tilting such a resiliently mounted cab, it is known to use a hydraulic tilting device with a tilting cylinder disposed between the chassis and the tilting cab. In order to ensure that while the vehicle is being driven the tilting cylinder does not interfere with the spring movements of the cab relative to the chassis, tilting devices with a so-called lost-motion mode are used. These tilting devices can be divided largely into two types, on the one hand, a mechanical type with, for example, a lost-motion arm, which is usually pivotably connected between the tilting cylinder and the cab, or with a sort of pin-and-groove connection between the tilting cylinder and the cab, and, on the other hand, a hydraulic type.
In the case of a tilting device with a hydraulic lost-motion, the tilting cylinder exhibits a lost-motion effect. A hydraulic cab tilting device of this type is known, for example from GB 2 079 378.
It is proposed in this publication that when the vehicle is being driven and the cab is carrying out spring movements, the piston/piston-rod assembly can move up and down unimpeded, since the piston is in a lost-motion range that is defined by the two ports of the lost-motion conduit. In this case the pump of the tilting device is out of action in this lost-motion mode.
In the case of this known tilting device anti-suction valves are provided in the fluid line system in such a way that in the lost-motion mode said valves ensure that fluid cannot be sucked out of the reservoir and cannot enter the cylinder space of the tilting cylinder. The result that is intended to be achieved with this in the case of the known tilting device is that the movement up and down of the piston/piston rod assembly in the lost-motion mode should lead to the displacement of a certain quantity of hydraulic fluid out of the tilting cylinder. The object of this is to create a vacuum in the cylinder space of the tilting cylinder, since the suction of fluid out of the reservoir is blocked. The object of the vacuum is to impede the movement up and down of the piston/piston rod assembly as little as possible in the lost-motion mode, so that the spring movement of the cab directly connected to the piston/piston rod assembly is not disrupted.
It appears from experiments that the tilting device proposed in GB 2 079 378 does not function as expected. In particular, it appears that the functioning of the lost-motion mode in the manner described in GB 2 079 378 cannot be guaranteed during the envisaged service life of such a tilting device. It is pointed out that during that service life the piston/piston rod assembly will move up and down many millions of times in the lost-motion mode.
It is in fact found that the known tilting device is susceptible to two phenomena.
The first phenomenon is that, taking into account the envisaged service life, there is a great chance that at a certain point in time the sealing effect of the piston rod seal will be inadequate, so that air will be able to enter the cylinder space. In this case the vacuum that prevails during the lost-motion mode has an adverse effect on that sealing effect.
The second phenomenon is that the valve in the lost-motion conduit is set to open at a certain hydraulic pressure and otherwise to seal off that lost-motion conduit. Again taking into account the envisaged service life, the functioning of this valve will also deviate from the envisaged functioning after some time, for example owing to almost unavoidable wear. In particular, there is a considerable chance of the valve at some point opening at a pressure that is lower than that intended.
As a result of these phenomena, it can happen that air is sucked in along the piston rod seal. Such air, once sucked in, remains in the hydraulic system of the tilting device. The air sucked in causes an excessively large quantity of fluid to be forced out of the tilting cylinder to the reservoir, by way of the lost-motion conduit and the push chamber, with the result that the pressure in said reservoir increases and the reservoir may overflow. The quantity of fluid forced out is particularly great if the opening pressure of the valve in the lost-motion conduit is lower than was originally envisaged, which can be the consequence of the almost unavoidable wear of that valve.
The air sucked in does not interfere greatly with the lost-motion effect of the tilting cylinder, but constitutes a problem in particular when the cab is subsequently being tilted by means of the tilting device. It can then, in fact, happen that the cab tips forward abruptly with an enormous bang as soon as the centre of gravity of the cab passes the pivot point relative to the chassis. The cause of that undesirable and potentially dangerous movement of the cab is the air present in the pull chamber of the tilting cylinder.
Another risk is that so much air is sucked in that hydraulic fluid flows out of the reservoir and a shortage of fluid arises. That shortage can be such that the tilting of the cab cannot be carried out properly or at all. There is also a considerable chance then of the shortage of hydraulic fluid being made up, but of the tilting device, in particular the pull chamber, being inadequately vented. The problem mentioned earlier can occur again as a result of this.